Coated release paper



United States Patent 3,403,045 COATED RELEASE PAPER Richard C. Ericksonand Robert C. Williams, Richmond,

Va., assignors to Albemarle Paper Company, Richmond, Va., a corporationof Virginia No Drawing. Continuation-impart of application Ser. No.286,453, June 10, 1963. This application Mar. 3, 1967, Ser. No. 620,2527 Claims. (Cl. 11768) ABSTRACT OF THE DISCLOSURE This invention isconcerned with the provision of paper sheets having markedly enhancedrelease properties by virtue of having a highly effective lamellarcoating affixed thereto, said coating consisting of a silicone polymercoating which is carried by a polyethylene or other comparable plasticundercoating.

CROSS REFERENCES TO RELATED APPLICATIONS This application is acontinuation-in-part of application Ser. No. 286,453, filed June 10,1963, now abandoned.

BACKGROUND OF THE INVENTION As is well known, the paper industry iscontinually called upon to provide paper products of enhanced utilityfor specific applications. In the packaging of tacky materials, such asrubber, asphalt, and the like, as well as in the production of pressuresensitive tapes, it is necessary to provide paper sheets having goodrelease properties. In the absence of suitable release properties, thepaper would adhere tenaciously to the tacky substance and cause obviousdisadvantages in use.

It is common practice to utilize organo polysiloxane coatings upon paperin order to provide suitable release properties. These organopolysiloxanes (commonly referred to as silicone polymers such as polydimethyl siloxane) have very good release properties and are essentiallyfree from the tendency to migrate to the tacky surfaces; however, thesesilicone coatings are very expensive. Furthermore, there is a limit tothe ability of organo polysiloxane coatings to afford optimum releaseproperties. Experience has shown that the application of commerciallyfeasible amounts of silicone coatings to kraft paper provides at best aKeil release value of 30 grams. Generally speaking, silicone coatings onkraft paper produce Keil release values ranging from 30 upwards to about300 grams. A significant contribution to the art would be the provisionof a kraft sheet having a Keil release value lower than 30 grams so thatthe article could be considered as a super release sheet. In thisconnection the standard Keil test measures release values according to astandard scale such that the lower the number the lower the adherence onor the better the release from the surface. Such a contribution would beparticularly noteworthy if it could be accomplished through the use ofreduced amounts of the expensive silicone coatings.

Accordingly, it is an object of this invention to provide paper sheetsof enhanced release properties. Another object is to provide a superrelease sheet, that is, kraft paper sheets having markedly enhancedrelease properties by virtue of having affixed thereto a highlyeffective lamellar coating which is capable of providing Keil releasevalues lower than 30 grams. A still further object is to provide novelarticles and methods for their manufacice ture characterized byexcellent release properties achieved at relatively low costs. A morespecific object is to provide means by which the amount of conventionalsilicone release coatings for kraft paper and the like can besubstantially reduced while at the same time obtaining better releaseproperties than heretofore feasible on a commercial basis. Otherimportant objects will be apparent from the ensuing description.

SUMMARY OF THE INVENTION In accordance with this invention, a lamellarcoating is applied to kraft paper and the like and especially to kraftpaper having a Gurley densometer value of at least 20 seconds per 100cc. Upon this paper is afiixed a coating of polyethylene, preferably inan amount equivalent on a dry basis to from about 1 to about 25 poundsper 3000 square feet. Superimposed and afiixed upon this polyethylenecoating is a cured organo polysiloxane coating, preferably in an amountequivalent on a dry basis to from about 0.05 to about 1 pound per 3000square feet. Methods and techniques for applying these respectivecoating layers are available in the art.

A highly significant feature of this invention is the discovery that theabove described lamellar coated kraft paper often possesses a Keilrelease value in the range of 10 to 15 grams even though only about 20percent as much silicone coating is used as compared with that amountheretofore necessary to obtain a Keil release value of 30 grams.Further, polyethylene itself has a Keil release value in the order of100' grams or more.

Hence, this invention may be viewed as interposing a layer of a materialhaving a relatively high Keil release value between kraft paper and asilicone coating which normally has at best a Keil release value of 30grams and thereby achieving a release value significantly lower than canbe achieved from practical amounts of either coating material when usedalone.

DESCRIPTION OF THE PREFERRED EMBODIMENT The porosity of the kraft paperused as the substrate material for the production of the release paperof this invention may vary over the wide range of porosities generallyfound in the production of papers. Release papers having the desiredproperties may be prepared from papers having a wide range of Gurleydensometer values; however, as stated previously, kraft papers having aGurley densometer value of at least 20 seconds per 100 cc. arepreferred. For best results, the kraft paper should have a Gurleydensometer value of at least about 50 seconds per 100 cc., and paperwith a densometer value of from about 50 to about 200 seconds per 100ccs. is especially preferred. It is also highly desirable to utilizekraft paper having a basis weight of from about 30 to about pounds per3000 square feet, although other grades may be utilized for specificapplications. It will be understood that kraft paper is utilizedprimarily because of its desirable physical characteristics andrelatively low cost. However, the principles of this invention may beapplied if desired to the utilization of other fibrous cellulosic sheetmaterial, such as bleached paper and the like as well as papers madefrom pulps prepared by chemical, mechanical or chemical-mechanicalprocesses other than the kraft process.

In particularly preferred embodiments of this invention the intermediatepolyethylene coating is affixed to the paper substrate in an amountequivalent on a dry basis to from about 2 to about 8 pounds per 3000square feet as this amount provides the best results at the lowest cost.For the same reasons the polysiloxane coating superimposed thereon ismost preferably utilized in an amount equivalent on a dry basis to fromabout 0.1 to about 0.2 pound per 3000 square feet.

Examples of particularly suitable articles provided by this inventioninclude kraft paper having a Gurley densometer value of from about 50 toabout 200 seconds per 100 cc., having aflixed thereto a coating ofpolyethylene in an amount equivalent on a dry basis to from about 2 toabout 8 pounds per 3000 square feet, upon which coating is affixed acured organo polysiloxane coating in an amount equivalent on a dry basisto from about 0.1 to about 0.2 pound per 3000 square feet, especiallywhen the kraft paper has a basis weight of from about 30 to about 70pounds per 3000 square feet.

To prepare the novel lamellar coated papers provided by this invention,the appropriate amount of polyethylene is applied to the kraft papersubstrate and thereafter an appropriate amount of the coating of theorgano polysiloxane is applied to the polyethylene layer and subjectedto a cure, preferably a heat cure. Depending upon the end use of thecoated article, the kraft paper substrate may be coated on one or bothof its sides in the manner described above.

To apply the polyethylene to the kraft paper, use may be made of suchtechniques as emulsion coating, solution coating, and like techniques,or, if desired, the polyethylene layer may be extruded upon the papersurface. Conventional commercially available grades of polyethylene,both high density and low density, may be employed. The main features ofthis step are to apply the polyethylene resin substantially uniformlyover the paper surface to be coated and to utilize for this purpose thepreferred amounts described above.

Application of the silicone coating likewise involves use of techniquesknown to those skilled in the paper coating arts. Hence, thematerial-usually in monomeric or partially polymerized form-can beapplied by such means as air-knife coating, trailing blade, hair brushcoater, and roll coater. For best results, the silicone coating which isthereby applied substantially uniformly upon the surface of thepolyethylene is dried in contact with a polymerization catalyst and thencured as quickly as possible so that the desired amount of polymerizedsilicone is held on and tightly aflixed to the surface. This is readilyaccomplished by drying the silicone coating-usually, but notnecessarily, applied as an emulsion containing the polymerizationcatalystby the use of drier ovens, hot air ovens, high velocity airdriers, infrared heaters, infrared lamps, or suitable combinations ofthese. When utilizing common commercially available silicone coatings inemulsion form an especially suitable cure condition is to expose thesame for 10 seconds to a temperature of 350 F. The use of highertemperatures will give shorter cure times. Further information anddetails relative to silicone coatings are well known to the art andreported in the literature, for example, in Rochow Chemistry of TheSilicones, second edition, John Wiley and Sons, Publisher (1951).

Generally speaking, the silicone bath formulations contain from about 1to about 10 percent by weight of silicone, preferably 7-10 percent.Examples of the silicones are dialkyl polysiloxanes, diarylpolysiloxanes, alkyl-aryl polysiloxanes, and mixtures thereof. Theformulations will further contain a conventional quantity of apolymerization catalyst to effect polymerization or furtherpolymerization of the silicone contained in the bath. Exemplary of suchcatalysts are zinc and tin soaps (such as zinc and tin octoate, zinc andtin stearate, zinc and tin oleate, etc.), alkyl tin carboxylates (suchas dibutyltin dilaurate, etc.), mixed stannous and stannicpolymerization systems, organic amines, or other soaps (e.g., ironoctoate, etc.), or the like. An example of such a formulation is asfollows:

Pounds Methyl silicone emulsion (40% solids) 65 Silicone polymerizationcatalyst (24% solids) 13 Car-boxymethyl cellulose as a viscositymodifier 10 Acetic acid to prolong pot life 4 Water 273 Exemplary of thepractice and advantages of this invention, sheets of various types ofkraft paper as described above were coated with polyethylene (2 poundsper 3000 square feet). This was accomplished by dissolving commerciallyavailable polyethylene in hot toluene (approximately 190 F.) andapplying this'solntion uniformly'over the surface of the paper sheets.After evaporating the toluene a roll coater and a rod doctor were.usedto apply the silicone from a 7 percent bath having the compositioncorresponding to that given above. This coating was subjected to a curetemperature of 550-600 F. for 4 seconds. The resultant lamellar coatedkraft sheets were found to possess Keil release values of 10-15 grams.

On a production basis the coating sequence of this invention may beeffected utilizing the same coating machine or the paper stock may bepassed into a first coating apparatus for application of thepolyethylene coating and thereupon be passed into a second coater forapplication of the silicone laminate.

While this invention has been discussed with particular reference to theutilization of polyethylene as the intermediate layer, the principlesand practice of this invention extend to the utilization of othercomparable plastics, such as polyvinyl chloride, polyvinylidenechloride, polypropylene, polyvinyl alcohol, polyvinyl acetate, or anysolutions, emulsions, or molecular dispersions which contain suchplastic materials. Likewise copolymers of ethyl- P ene, propylene, vinylchloride, vinylidene chloride, vinyl alcohol, vinyl acetate, and thelike may be used. Mixtures of the foregoing may also be used. For mostapplications polyethylene is most suitable because of its abundance,ready availability, relatively low cost, and excellent releaseproperties when utilized pursuant to this invention. As will be apparentto those skilled in the art, the coating solutions, emulsions, ormolecular dispersions may also contain extenders, such as clay, pigment,or fillers, the use of which will be dependent to some extent on theultimate use to which the end product is to be put.

Accordingly, the novel and useful contribution constituting the presentinvention is as defined in the ensuing clfaims and extends to the fullrange of equivalents there- 0 What is claimed is:

1. In a release paper having a release type silicone polymer coat1ngimparting release characteristics to the paper, the mprovement accordingto which the silicone polymer coating is carried by a polyethyleneundercoating weighmg about 1 to about 25 pounds per 3000 square feet ofthe paper.

2. The combination of claim 1 in which the paper is kraft paper.

3. The combination of claim 1 in which the paper has a basis weight offrom about 30 to about 70 pounds per 3000 square feet.

4. The combination of claim 1 in which the paper has a Gurley densometervalue of at'least 20 seconds per cc.

5. The combination of claim 1 in which the pair of coatings isduplicated on'both faces of the paper.

6. The combination of claim 1 in which the polyethylene weighs fromabout 2 to about 8 pounds per 3000 square feet and the silicone polymercoating Weighs from about 0.1 to about 0.2 pound per 3000 square feet.

7. The combination of claim 6 in which the paper is kraft paper having aGurley densometer value from about 50 to about 200 seconds per 100 cc.and a basis weight from about 30 to about 70 pounds per 3000 squarefeet.

(References on following page) References Cited UNITED STATES PATENTSLewis et a1. 117155 Meyer-Jagenberg 117155 X Wilkins 117155 X Eder eta1. 117155 X 6 OTHER REFERENCES Meals et a1.: Silicones, ReinholdPublishing Corp., New York, 1959, TP 248.55 M4 (pages 128-134 reliedon).

5 WILLIAM D. MARTIN, Primary Examiner.

R. HUSACK, Assistant Examiner.

